June 2016 Issue

Foreign Service

By Dan Marinucci

An ECM may mistakenly blame a system leak for low evap vacuum, with DTCs to match. Identifying the real cause requires system knowledge and focused testing.

Practical experience and trouble codes determine the outcome of many repair jobs. However, some technicians take this conclusion to extremes; namely, they believe that vehicle familiarity and a DTC always ensure success. But often these two elements may be just baby steps toward an accurate analysis. Thorough troubleshooting often requires several more steps—steps that lead to a job fixed right the first time.

This month’s topic is an elusive DTC on a Honda Civic. It illustrates the difference between stubbornly relying on a combination of experience and a DTC vs. actually diagnosing a problem. This Civic saga comes courtesy of Brandon Steckler at Terry Wynter Auto Service Center in Fort Myers, FL. Steckler explained how a previous employer tasked him with fixing the cause of a P1457 code in a 2005 Civic. Other techs in the shop had worked on the car, but this DTC kept coming back.

If you’re unfamiliar with a common Honda evaporative emissions setup, it has a typical purge solenoid valve at the front of the system and a canister vent solenoid at the back end. The ECM cycles or pulses the purge solenoid valve during routine purging. The canister vent solenoid, which Honda guys usually call a canister vent shut valve, is either electrically on or off. Mechanically, it’s either open or closed.

However, Honda’s evap setup differs from the domestic systems because it has two components separating the canister side from the fuel tank side of the system. One device is the bypass solenoid valve that’s either electrically on or off. Mechanically, it’s either open or closed. The ECM energizes the bypass solenoid valve when it runs the evap monitor. Then the bypass valve opens, connecting the tank side of the system with the canister side during the monitor’s leak check. What’s more, the P1457 code suggests a leak on the canister side of the evap system; P1456 points to a leak on the fuel tank side.

Honda’s other component is a fully mechanical device called a two-way valve; it’s plumbed parallel to the bypass valve. The two-way valve prevents extreme pressures from occurring inside the fuel tank.

Let’s return to the problem car. The details Steckler knew for certain were that the gas cap and canister shut solenoid valve had been replaced. First, Steckler used a scan tool to exercise the solenoids in the evap system. Each of the three solenoids operated normally on command. Next, he energized both the bypass solenoid valve (opening it) and the canister shut solenoid valve (closing it). Then he checked the system for leaks with a smoke machine. No leaks occurred.

Furthermore, a quick measurement confirmed that the engine was pulling a healthy manifold vacuum on the engine side of the purge solenoid valve. So Steckler learned several vital things within a short time. First, the engine was supplying normal vacuum to the “front door” of the entire evap system. Second, none of the evap solenoids were stuck at the time. Third, no obvious leaks were present at the time. Yet, the ECM believed that the canister side of the system was leaking.

By this point, Steckler suspected a purge problem of some kind. After all, inadequate purging during the evap monitor would prevent the system from dropping into the proper vacuum range. Then the ECM might mistake this inadequate vacuum for a leak instead of insufficient purging.

Next, Steckler used his scan tool to command an evap self-test (sometimes called a “service-bay” evap check). Among several functions, this self-test energizes the canister shut solenoid, closing the “back door” of the evap system. It energizes the bypass solenoid, opening the bypass valve. Then it pulses the purge solenoid valve at idle. The end result is that the entire evap system should drop into a vacuum during the test.

Steckler monitored the fuel tank pressure (FTP) sensor signal during the test. At atmospheric pressure, the sensor’s output signal is approximately 2.50V. This signal should drop below 2.50V when the evap system goes into a vacuum.

This is where experience paid off in a positive sense. Based on dozens of previous evap tests, Steckler knew that a healthy FTP sensor signal dropped noticeably and steadily. On this Civic, however, the FTP signal initially dropped a little bit—but then the falling signal slowed almost to a standstill! Surely, this was an abnormal response.

Steckler’s next tactic was to temporarily eliminate the purge solenoid valve from the system. He did this by removing the purge hose from the canister side of the purge valve. Then he substituted his own vacuum by connecting a hand-held vacuum pump to that purge hose. Next, he energized both the bypass and the canister shut solenoids. With the bypass valve open and the canister shut valve closed, he expected to be able to “vacuum down” the entire evap system with the hand-held pump.

Now the FTP sensor signal reacted promptly and properly to the lowered pressure inside the system. In fact, Steckler’s hand pump lowered evap pressure enough to drop the FTP sensor signal to 1.50V. Next, he clamped off the purge hose just downstream of the vacuum pump hookup; the FTP signal remained rock steady. These tests yielded two more clues that the Civic’s evap system really wasn’t leaking and its FTP sensor worked normally.

Steckler’s last check was to be twofold: Check purge solenoid operation independently of the ECM and scope-test it at the same time. To do this, he backprobed the ground side of the purge solenoid and connected a test lead to it. Then he restarted the engine and momentarily grounded the test lead. Steckler scoped the current flowing through the solenoid by clipping a low-current probe around the grounding lead. Typically, pintle movement inside a solenoid creates a noticeable bump in the amps pattern; this bump usually appears about two-thirds of the way up the pattern’s rising slope.

However, no pintle bump appeared on the original purge solenoid’s scope pattern. Usually, this indicates sticky or sluggish solenoid operation. Meanwhile, replacing the purge solenoid valve eliminated the P1457 code for good. What’s more, the scope pattern for the new solenoid had a normal pintle bump. If this approach sounds familiar, it’s the same one some techs use to analyze current flow through fuel injectors.

To recap, the failing purge solenoid valve did duty cycle normally when the evap system was being purged. Its duty cycle reading was normal; it clicked normally and consistently. An ohmmeter measurement showed that its resistance was only slightly different from that of the brand-new solenoid. The solenoid valve did not leak vacuum or smoke. But a quick scope check showed an abnormal pintle movement—that step nailed the car’s problem.

In conclusion, it’s nothing new for an ECM to mistake abnormally low evap vacuum for a leak when the root cause is inadequate purging. But the difference in this case was a tech who took extra steps to validate the real cause—instead of blindly following the leak-related DTC. We salute techs like Steckler for their professionalism.